In recent years, significant development work has been done in the field of ink jet printing. One type of ink jet printing involves electrostatic, pressurized ink jet, wherein conductive ink is applied under pressure to a suitable nozzle or nozzles. The ink is thus propelled from each nozzle in a stream which is caused to break up into a train of individual droplets which must be selectively charged and controllably deflected for recording or to a gutter. The droplet formation may be controlled and synchronized by a number of different methods available in the art including physical vibration of the nozzle, pressure perturbations introduced into the ink supply at the nozzle, etc. The result of applying such perturbations to the ink jet is to cause the jet stream emerging from the nozzle to break into uniform droplets at the perturbation frequency and at a predetermined distance from the nozzle. It is of utmost necessity in such systems to precisely synchronize the application of the appropriate charging signal to the ink droplet stream at the precise time of droplet formation and breakoff from the stream. Means for supplying the selected electrostatic charge to each droplet produced by the nozzle conventionally comprises a suitable charging circuit and an electrode surrounding or adjacent to the ink stream at the location where the stream begins to form such droplets. Charging signals are applied between a point of contact with the ink and the charging electrode. A drop will thus assume a charge determined by the amplitude of the particular signal on the charging electrode at the time that the drop breaks away from the jet stream. The drop thereafter passes through a fixed electric field and the amount of deflection is determined by the amplitude of the charge on the drop at the time it passes through the deflecting field. In the binary type of electrostatic ink jet, uncharged drops are not deflected and proceed directly to a recording surface positioned downstream from the deflecting means such that each such drop strikes the recording surface and forms a small spot. Charged drops are deflected by the deflecting means to a gutter. U.S. Pat. No. 3,373,437 of Richard G. Sweet et al entitled "Fluid Droplet Recorder with a Plurality of Jets" discloses such a recording or printing system.
The time that the drop separates from the fluid stream emerging from the nozzle is quite critical, since the charge carried by the droplet is produce at that moment by electrostatic induction. The field established by the charging signal is maintained during drop separation, and the drop will carry a charge determined by the instantaneous value of the signal at instead breakoff. Thus, the droplet breakoff time and the application of the charge signal must be very precisely synchronized.
In the binary type of system, if synchronization is not correct such that the charging signal is in the process of either rising or falling at the time of drop breakoff, the exact charge of the drop will be some time function of the maximum charge signal rather than being fully charged. Such drops may be deflected by an amount too small to cause impact with the gutter, but instread would impact the recording medium at an unintended position.
The binary type of system normally employs a series of small nozzle orifices in a single ink jet head. Although great care may be exercised in attaining precise parallel directionality of the jets, partial clogging or crusting of any nozzle orifice will alter the directionality, resulting in erroneous spot placement and attendant degradation of print quality.
With respect to the problem of obtaining proper synchronization between the charged signal and drop breakoff, the prior art definitely recognized the criticality of the synchronization problem and many sensors have been proposed for testing the drops for proper charging and to allow adjustment of the synchronization between the charging signals and the perturbation means. The following U.S. patents are representative of the prior art: Keur et al., U.S. Pat. No. 3,465,350; Keur et al., U.S. Pat. No. 3,465,351; Lovelady et al., U.S. Pat. No. 3,596,276; Robertson U.S. Pat. No. 3,761,941; Hill et al. U.S. Pat. No. 3,769,630; Julisburger et al. U.S. Pat. No. 3,769,632; Ghougasian et al. U.S. Pat. No. 3,836,912; Carmichael et al. U.S. Pat. No. 3,852,768; Meier U.S. Pat. No. 3,866,237; Naylor et al. U.S. Pat. No. 3,886,564; and Haskell U.S. Pat. No. 3,898,673.
The Keur et al, U.S. Pat. No. 3,465,350 describes the use of a piezoelectric member which generates a signal in response to drop impact. The Keur et al. U.S. Pat. No. 3,465,351, the Lovelady et al. patent, the Robertson patent and the Hill et al patent all disclose sensing electrodes where charged drops impacting the electrodes give up their charge thereto, which is sensed. The Meier patent and Haskell patent discloses the use of a segmented gutter having electrodes separated by a small gap to sense the resistance or conductance of the ink flowing at the gap. The Julisberger et al. patent, the Ghougasian et al. patent and the Carmichael et al patent all describe single shielded induction sense electrodes having an aperture through the shields for passage of an ink jet drop stream for induction sensing charged drops. The Charmichael et al. patent also discloses a shielded probe for placement adjacent an ink jet drop stream for induction sensing charged drops. The Naylor et al. patent describes an induction sensor comprising two plates lying in the same plane parallel to an ink jet drop stream and separated by a small gap to sense the plate by which the stream is passing or is closest.
All of the foregoing art is primarily directed to sensors for detecting a single drop stream and for sensing a single characteristic of that drop stream.
It is therefore an object of the present invention to provide a common antenna structure for a plurality of parallel drop streams adapted to be employed to sense a plurality of characteristics of each stream.